This project seeks to provide a better understanding of the secretory immune system. The reasons underlying the predominance of IgA plasma cells in mucous membranes and of the IgA class of antibodies in exocrine secretions will be studied. The hypothesis that lymphocytes with dimeric IgA on the surface home preferentially to mucous membranes will be tested by trying to inhibit homing by prior treatment of the cells with anti-J chain antiserum, or by simultaneous injection of mouse erythrocytes coated with dimeric IgA. In order to understand how IgA reaches milk, we shall try to develop a model of cellular homing to the mammary glands of pregnant and lactating mice, or of virgin mice treated with exogenous hormones. We shall study the requirements for uptake of radioiodinated IgA by suspensions of epithelial cells from intestine, and test whether uptake can be blocked by antibody to secretory component, under the hypothesis that this protein is the natural receptor mediating uptake. Since final assembly of secretory IgA occurs within epithelial cells, we shall study whether an enzyme which promotes disulfide-exchange can catalyze the association of secretory component with IgA. We also want to know the particular cysteine residues in secretory IgA to which secretory component is covalently bound in the secretory IgA molecule.